eyeglass-exports: segmentation fault / abort trap

Nobody has claimed this yet.

I read through #69 and #74, which seem to be the closest to this, but I don't think are the same.

My setup

```js
// @fastly/fui-color/package.json
"eyeglass": {
"exports": "lib/eyeglass-exports",
"sassDir": ".",
"needs": "*"
},

// @fastly/fui-color/lib/eyeglass-exports.js
module.exports = function(eyeglass, sass) {
return {
functions: {
'fastly-color($name)': function($name, done) {
done(sass.types.String('#abcdef'))
}
},
}
}
```
I'm including that in an ember app with
```
node@10.13.0
broccoli-eyeglass@4.4.4
ember-cli@3.1.3
ember-cli-eyeglass@5.0.1
eyeglass@1.6.0
node-sass@3.13.1
```

When I run `ember build`, it fails with
```
Assertion failed: (!persistent().IsWeak()), function Unref, file ../../../../nan/nan_object_wrap.h, line 107.
Abort trap: 6
```

If I upgrade to node-sass@4.10.0, it instead fails with
```
libc++abi.dylib: terminating with uncaught exception of type std::length_error: vector
Abort trap: 6
```

Start by reproducing the failure from lib/eyeglass-exports.js with the listed Node, broccoli-eyeglass, Ember CLI, eyeglass, and node-sass versions, then compare the ember build errors for node-sass 3.13.1 and 4.10.0. Done means the eyeglass export builds without a segmentation fault or abort trap, with the relevant compatibility cause documented or covered by a regression test.

Written by the indexing model from the issue text.